Hitherto, a coating method for decorating has been often used for improving decorativeness of a resin product having a base material composed of a thermoplastic resin.
With the known coating method, in general, after a formed product subjected to injection molding in a mold is taken out of the mold, a coating material is applied on the surface of the formed product by a spray method, an osmotic method, or the like. Then, by drying and curing the applied coating material, an unbreakable film is formed and the surface of the formed product is coated, so that the surface is decorated and protected.
However, in recent years, an in-mold coat-forming method (sometimes, referred to as an in-mold coating method) with which the forming with a resin and film coating are performed in the same mold has been proposed in order to eliminate a process of the above coating method.
FIG. 14 is a flowchart illustrating a general process of the in-mold coat-forming method. According to the in-mold coat-forming method shown in FIG. 14, after a resin is subjected to injection molding in a mold, the mold is slightly opened so as to form a clearance between the resin formed product formed in the mold and the surface of a mold cavity. Then, after a coating material is injected into the clearance by using a coating-material injector, the coating material is uniformly extended over the surface of the formed product by clamping the mold again and is then cured so as to coat the surface.
According to the in-mold coat-forming method, since the forming with a thermoplastic resin and the coating thereon are carried out in the same mold, a cost reduction by eliminating one process can be achieved, and at the same time, a problem in that suspended dust accretes on an uncured film and causes a defective product, and the like, seldom occur, thereby achieving a high-quality product.
Thus, an application of the foregoing in-mold coat-forming method for many parts, especially for automobile parts including a bumper, a side-mirror cover, and a fender which are required to have high-quality appearance, has been studied.
Meanwhile, many in-mold coat-forming methods are carried out by using a mold having a shear-structured engaging portion extending along the circumference of the mold cavity in order to prevent leakage of a coating material.
However, when a coating material is actually injected, sometimes the coating material leaks out of the mold cavity through the shear-structured engaging portion (sometimes, referred to as the shear portion), and as the number of usages of the mold increase, the shear portion is more worn away, causing the clearance of the shear portion to gradually increase and thus the coating material to be more likely leaked.
Meanwhile, sometimes the shear portion is generally called a shear-edge structure.
In order to prevent the above problem, an in-mold coat-forming mold with which leakage of a coating material is prevented by disposing an auxiliary cavity along the circumference of the mold cavity and by filling a coating material in the auxiliary cavity has been proposed in JP-A-2001-138334.
Although this mold has an excellent advantage in that a coating material is unlikely to leak even when its share portion is worn away, the coating material sometimes leaks due to shrinkage of a resin filled in the auxiliary cavity.
A cause of the leakage of a coating material when the known mold having the foregoing auxiliary cavity is used will be conceptually described with reference to FIGS. 5(a) to (c). Firstly, when a resin filled in an auxiliary cavity 1 as shown in FIG. 5(a) is cooled and shrinks thermally before the mold is slightly opened as shown in FIG. 5(b), a slight gap is formed between the auxiliary cavity 1 and the surface of the mold. This gap causes the coating material to leak out as shown in FIG. 5(c).
JP-A-9-48044 has disclosed an in-mold coating method using an injection mold which includes means for injecting a coating agent into the injection mold in order to apply a coating on the surface of a synthetic resin-formed product in the injection mold and which is characterized in that the injection mold is formed by at least two dividable mold members, the two mold members have an auxiliary cavity disposed in a parting surface thereof along the circumference of a main cavity so as to communicate with the main cavity, and a grooved thread for preventing the coating agent from flowing out is disposed in the auxiliary cavity so as to lie on the side of the surface through which the coating agent is injected, of the mold member, characterized in that, after a thermosetting resin or a thermo plastic resin is subjected to injection molding in the cavity, a mold-clamping force is decreased, and, when the surface of the resin is properly cured or solidified so as to stand against the injection-flowing pressure of the coating agent, the coating agent is injected in the boundary between the inner surface of the injection mold and the resin formed product with a higher pressure than the mold-claming force. However, the method disclosed in the Laid-open Patent Application has not disclosed an in-mold coat-forming method with which a coating material is injected after the mold is slightly opened in a positive manner.
In other words, the above method has disclosed that the mold-clamping force is decreased after a resin is subjected to injection molding; then, when the resin is cured or solidified to a degree to which the resin withstands against the injection-flowing pressure of the foregoing coating agent, by injecting the coating agent with a higher pressure than the clamping force, the coating agent causes the resin in the cavity to be compressed and also the mold to be detached therefrom; and the grooved thread formed in the auxiliary cavity receives the coating agent so as to effectively prevent it from flowing out from the parting surface caused by opening the mold. However, the above Laid-open Patent Application has not disclosed at all that a groove is disposed in the engaging portion or in the vicinity of the edge of the cavity adjacent to the engaging portion.